The present invention relates to flanges for turbomachines, such as steam turbines and the like. More specifically, the present invention relates to a spreading plate that distributes the bolt loading over a steam turbine horizontal joint flange.
A turbomachine, such as a steam or gas turbine or the like, is comprised of a stationary cylinder that surrounds a centrally disposed rotor. Typically, the stationary cylinder is split horizontally into upper and lower halves to allow the rotor to be installed into the cylinder. The upper and lower halves are joined along mating horizontal flanges by bolts--as used herein the term "bolts" refers to any threaded fastener, including screws and studs. In one design, the bolts are threaded into tapped holes in the flanges in the lower cylinder half and nuts are threaded onto the free ends of the bolts that extend above the flanges in the upper cylinder half. To ensure that the pressurized working fluid--i.e., steam in the case of a steam turbine or gas and air in the case of a gas turbine--does not leak through the horizontal joints, a substantial preload is applied to the bolts.
As a result of the high bolt preload, the stresses in the bolt threads are very high. To ensure that the nut does not seat improperly, thereby imposing additional stress on the bolt due to the presence of a bending moment, in the past, a two piece washer was disposed between the nut and the flange. The upper washer piece had a convex spherical surface that mated with a concave spherical surface in the lower washer piece. The spherical surfaces allowed the upper washer piece to rotate under load so as to provide a surface for the nut to bear against that was perpendicular to the axis of the bolt.
In addition to the danger of excessive bolt stress, the high bolt preload can cause local creep in the portions of the flange under the nuts. This problem arises because the aforementioned washers distributed the bolt load over only a very small area of the flange surrounding each bolt hole. As a result, the localized contact stresses in the portions of the flange surface under the washer were very high. Consequently, after sufficient operating time at elevated temperature, local compressive creep occurred in the flange under the washers--referred to as creep indentation--that locally reduced the thickness of the flange causing a reduction in the bolt preload. This problem is exacerbated by thermal transients that cause differential thermal expansion between the bolts and the cylinder flange. Such differential thermal expansion can temporarily increase the bolt loading and, as a result, increase the local compressive stress in the portions of the flanges under the nuts beyond the yield point, leading to local crushing of the flange and further permanent loss of bolt preload. After sufficient operating time, the loss of bolt preload from these two mechanisms can result in leakage at the horizontal joints.
One approach considered in the past for solving this problem is to use a cylinder material that has very high yield strength and hardness. Unfortunately, although such high yield strength materials would make good flanges, they have insufficient ductility for use in the remainder of the cylinder, which is subjected to a variety of thermal and cyclic stresses.
Another approach considered in the past is to cool the flange area of the cylinder by directing a cooling fluid over the area. Such cooling serves both to reduce the temperature of the flange below that at which creep is likely to occur and to reduce the temperature difference between the bolts and the flange so that transient thermal differential expansion does not become great enough to result in yielding of the flange. Unfortunately, the use of a cooling fluid--such as low pressure steam in a steam turbine or compressor air in a gas turbine--degrades the thermodynamic performance of the turbomachine.
It is therefore desirable to provide an apparatus for preventing creep indentation and localized yielding of the horizontal joint flanges of a turbomachine in the portions of the flanges under the joint fasteners.